U.S. patent number 5,342,431 [Application Number 07/987,891] was granted by the patent office on 1994-08-30 for metal oxide membranes for gas separation.
This patent grant is currently assigned to Wisconsin Alumni Research Foundation. Invention is credited to Marc A. Anderson, Elizabeth T. Webster, Qunyin Xu.
United States Patent |
5,342,431 |
Anderson , et al. |
August 30, 1994 |
Metal oxide membranes for gas separation
Abstract
A method for permformation of a microporous ceramic membrane
onto a porous support includes placing a colloidal suspension of
metal oxide particles on one side of the porous support and
exposing the other side of the porous support to a drying stream of
gas or a reactive gas stream so that the particles are deposited on
the drying side of the support as a gel. The gel so deposited can
be sintered to form a supported ceramic membrane having mean pore
sizes less than 30 Angstroms and useful for ultrafiltration,
reverse osmosis, or gas separation.
Inventors: |
Anderson; Marc A. (Madison,
WI), Webster; Elizabeth T. (Madison, WI), Xu; Qunyin
(Plainsboro, NJ) |
Assignee: |
Wisconsin Alumni Research
Foundation (Madison, WI)
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Family
ID: |
27411476 |
Appl.
No.: |
07/987,891 |
Filed: |
December 7, 1992 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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756395 |
Sep 9, 1991 |
5269926 |
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654150 |
Feb 11, 1991 |
5169576 |
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425668 |
Oct 23, 1989 |
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Current U.S.
Class: |
95/45; 96/10;
96/4 |
Current CPC
Class: |
B01D
67/0041 (20130101); B01D 67/0048 (20130101); B01D
67/0088 (20130101); B01D 71/024 (20130101); B01J
8/009 (20130101); B01J 19/2475 (20130101); B01J
35/065 (20130101); B01J 37/0211 (20130101); C04B
35/46 (20130101); C04B 35/486 (20130101); C04B
35/49 (20130101); C04B 35/624 (20130101); C04B
41/4537 (20130101); C04B 41/81 (20130101); B01D
2323/12 (20130101); B01J 2208/00017 (20130101); B01J
2208/00548 (20130101); B01J 2208/00557 (20130101); B01J
2208/00575 (20130101); B01J 2208/00592 (20130101); B01J
2208/0061 (20130101); B01J 2219/00051 (20130101); B01J
2219/00164 (20130101); B01J 2219/00177 (20130101) |
Current International
Class: |
B01J
19/24 (20060101); B01J 8/00 (20060101); B01D
71/00 (20060101); B01D 71/02 (20060101); B01J
35/00 (20060101); B01J 37/00 (20060101); B01J
37/02 (20060101); B01J 35/06 (20060101); C04B
35/49 (20060101); C04B 35/622 (20060101); C04B
35/624 (20060101); C04B 35/486 (20060101); C04B
35/46 (20060101); C04B 41/81 (20060101); C04B
41/45 (20060101); B01D 053/22 () |
Field of
Search: |
;55/16,68,158,524 ;95/45
;96/4.10 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0362898 |
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Apr 1990 |
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EP |
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0458217 |
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Nov 1991 |
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EP |
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3918190 |
|
Dec 1990 |
|
DE |
|
2177881 |
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Nov 1973 |
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FR |
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55-119420 |
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Sep 1980 |
|
JP |
|
57-207533 |
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Dec 1982 |
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JP |
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59-177117 |
|
Oct 1984 |
|
JP |
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PCTUS/8802537 |
|
0000 |
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WO |
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WO89/00983 |
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Feb 1989 |
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WO |
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0604826 |
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Sep 1978 |
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CH |
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Other References
Asaeda, M. and Du, L. D., "Separation of Alcohol/Water Gaseous
Mixtures by Thin Ceramic Membrane," J. Chem. Eng. Japan 19[1]:72-77
(1986). .
Anderson, et al., 39 J. Memb. Sci., 243-258 (1988). .
Chen, K. C., et al., "Sol-Gel Processing of Silica: 1. The Role of
the Starting Compounds," J. Non-Crystalline Solids 81:227-237
(1986). .
Coplan, M. J., "Endotreating: A New Approach to Composite
Membranes," Paper presented at 8th Annual Membrane
Technology/Planning Conference, Oct. 15-17, 1990, Newton, Mass.
.
Johnson, D. W., "Sol-Gel Processing of Ceramics and Glass," Am.
Ceram. Soc. Bull. 64[12]:1597-1602 (1985). .
Kamiya, K. et al., "Preparation of TiO.sub.2 Fibers by Hydrolysis
and Polycondensation of Ti(O-i-C.sub.3 H.sub.7).sub.4 ", J. Chem.
Eng. Japan 19[1]:72-77 (1986). .
Leenaars, A. F. M. and Burggraar, A. J., "The Preparation and
Characterization of Alumina Membranes with Ultra-Fine Pores Part 4.
Ultrafiltration and Hyperfiltration Experiments," J. Memb. Sci.
24:261-270 (1985). .
Yoko,, T. et al., "Photoelectrochemical Properties of TiO.sub.2
Films Prepared by the Sol-Gel Method," Yogyo-Kyokan-Shi 95[2]:
13-17 (1987). .
Yoldas, B. E., "A Transparent Porous Alumina," Am. Ceram. Soc.
Bull. 54[3]: 286-280 (1975). .
Yoldas, B. E., "Alumina Sol Preparation from Alkoxides," Am. Ceram.
Soc. Bull. 51[3]:289-290 (1975). .
Yoldas, B. E., "Preparation of Glasses and Ceramics from
Metal-Organic Compounds," J. Mater. Sci. 12:1203-1208 (Jun.
1977)..
|
Primary Examiner: Spitzer; Robert
Attorney, Agent or Firm: Quarles & Brady
Government Interests
This invention was made with United States Government support
awarded by the Department of Energy (DOE), Grant Nos.
DE-AS07-86ID26 and DE-FC07- 88ID12778, the Environmental Protection
Agency (EPA), Grant No. R813457-01-1; and the National Science
Foundation (NSF), Grant No. CES-8504276. The United States
Government has certain rights.
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application is a continuation-in-part of application Ser. No.
07/756,395 filed Sep. 9, 1991, now U.S. Pat. No. 5,269,926, and of
application Ser. No. 07/654,150 filed Feb. 11, 1991, now U.S. Pat.
No. 5,169,576, which was a divisional of application Ser. No.
07/425,668 filed Oct. 23, 1989, now abandoned.
Claims
We claim:
1. A process for separating gases, comprising the steps of:
(a) providing a microporous metal oxide ceramic membrane performed
in the pores of a porous support and having pores averaging less
than 40 Angstroms in diameter;
(b) exposing a mixture of gases of various molecular size and shape
against one side of the permformed microporous ceramic membrane
under conditions which favor the flow of gases through the
membrane;
(c) excluding one or more species of gas from passage through the
membrane because of the size of the pores of the microporous
ceramic membrane; and
(d) recovering at least one partially purified gas which has passed
through the membrane.
Description
FIELD OF THE INVENTION
The present invention relates to the general field of porous
ceramic membranes and relates, in particular, to a method for
depositing metal oxide ceramic membranes having very small pore
sizes onto porous supports, and the products produced by the
method.
BACKGROUND OF THE INVENTION
Porous ceramic membranes are durable film materials having a
variety of industrial and scientific uses, the most common of which
is use in separation processes. Although organic membranes are
currently used most often for industrial separation processes,
metal oxide ceramic membranes offer several advantages over organic
membranes. Metal oxide ceramic membranes have a greater chemical
stability, since they are resistant to organic solvents, chlorine,
and extremes of pH, to which organic membranes may be susceptible.
Ceramic membranes are also inherently more stable at high
temperatures, to allow efficient sterilization of process equipment
not possible with organic membranes and to allow for operation at
these elevated temperatures, e.g., above 200.degree. C., at which
no organic membrane can function. Metal oxide ceramic membranes are
also entirely inorganic, so they are generally quite stable and
resistant to microbial or biological degradation which can
occasionally be a problem with organic membranes.
The nature of the material results from the general procedure for
making such membranes. Metal oxide ceramic membranes are formed
through a process beginning with organic-inorganic molecules which
are formed into small metal oxide particles, then fused into a
unitary ceramic material. On a microscopic level, the materials may
be conceptualized as a series of generally uniform spherical
particles which are arranged in a close packing model, with the
junction points between the spherical particles being fused
together. The result is a durable inorganic, homogenous, amorphous
to crystalline material which has a relatively uniform distribution
of pores, with the pores being determined by the size of the
particles forming the membrane. The gaps between the fused
particles form a series of pores so that the membrane is porous.
The smaller the size of the particles, the smaller the holes left
between them, when the particles are packed together and fused.
The general approach to the manufacture of metal oxide ceramic
membranes consists of a sol-gel process. In the sol part of the
process, a dilute colloidal solution or suspension of metal oxide
particles is created. The metal oxide is typically initiated into
the process as a metal alkoxide dissolved in an alcohol solvent.
The introduction of the metal alkoxide to water with rapid stirring
results in the hydrolysis of the metal to metal hydroxide monomers,
dimers, polymers, and/or particles, depending on the quantity of
water used. Insoluble metal oxide particles are then peptized by
the addition of an acid which causes the particles of the metal
oxide to have a greater propensity to remain in suspension,
presumably due to charges acquired by the particles during the
peptizing process. This process is one of charge stabilization.
Stabilization could also be accomplished sterically by adding
surfactant agents. Care must be taken at this stage to prevent
accretion of large particles, if a small pore size is desired in
the membrane. Alternatively, an aqueous sol may be produced by
hydrolyzing a metal alkoxide or a metal salt.
Then, under very tightly controlled conditions, the alcohol or
aqueous solvent is removed from the colloidal sol, resulting in a
semi-solid phase of material known as a xerogel or gel. The gel is
typically a translucent or transparent semi-solid material which
will retain its shape, but is still relatively deformable. Removal
of the remaining water and solvent, and sintering of the gel
results in a durable rigid ceramic material which can either be
formed as an unsupported membrane or as a supported membrane coated
onto a substrate, which, in turn, can be either porous or
non-porous, and metallic or non-metallic, depending on the
particular application.
One desirable metal element for use in such a metal oxide ceramic
membrane is titanium. Titanium is attractive since it has catalytic
and photocatalytic qualities that make a titanium oxide ceramic
membrane useful for chemical or photoelectrochemical processes in
which a less catalytic or photocatalytic metal oxide ceramic
membrane would not be suitable. Also, titanium oxide ceramic
membranes are typically transparent or lightly colored, thereby
giving them desirable optical properties for certain applications
in which transparency is an asset.
Practical limitations on the use of such metal oxide ceramic
membranes have included the absolute size and the range of size of
the pores which can be created in the metal oxide membranes.
Clearly, if a membrane is to be used for filtration or other form
of separation, the size and the variance in size of the pores
through the membrane are a critical factor in the suitability of
the membrane for the particular separation function desired. There
must be limitations on the heat of the sintering process, since too
high a temperature will destroy the pores, but, within a wide
range, a porous ceramic material can be created as a supported or
as an unsupported membrane.
At least one teaching is known, by the inventors here, of a method
for preparing polymeric or particulate titanium ceramic membranes
by a process which allows the reproducible and predictable
fabrication of titanium ceramic membranes and which permits
crack-free membranes of predictable qualities to be created. As
disclosed in international published PCT patent application WO
89/00983, the method for creating particulate ceramic membranes
involved the use of relatively large amounts of water and a mild
heating during the peptizing step to create the appropriately
charged particles which could then be dewatered and sintered to
create a titanium oxide ceramic membrane.
The method for creating the polymeric ceramic membranes included
strictly limiting the amount of water included in the reaction
vessel so as to foster the creation of covalent bonds between the
titanium and oxygen molecules in the suspension, and also required
the use of an alkyl alcohol different from the alkyl alcohol in the
titanium alkoxide for the process to be effective.
Certain attention has been directed toward the creation of porous
ceramic membranes with exceedingly small pore size. An example of
such research is disclosed in U.S. Pat. No. 5,006,248. Similar work
is described in Anderson et al., Journal of Membrane Science, 39,
pp. 243-458 (1988). The process described in the above patent
enables the creation of porous ceramic membranes with small pore
sizes, either as supported or unsupported materials.
Metal oxide ceramic membranes of transition metals can also be used
for catalytic purposes. U.S. Pat. No. 5,035,784 describes how such
materials can be used under ultraviolet light to degrade
polychlorinated organic chemicals. Doping can be utilized in mixed
membrane materials to increase electrical conductivity for various
catalytic purposes. U.S. Pat. No. 5,028,568 describes the doping of
titanium membranes with niobium to achieve increased electrical
conductivity.
Practical utility of ceramic membranes requires large, thin,
crack-free surfaces which can be difficult to reliably make in the
unsupported form, due to the frailty of the ceramic material.
Therefore, supported membranes are more practical for most
applications. Traditionally, the accretion or layering of such very
small size ceramic particles onto a porous substrate has turned out
not to be a trivial endeavor. Such particles tend to accrete, or
deposit, on a substrate in an irregular manner resulting in
nonhomogeneous thickness. The pores of the substrate which the
microporous membrane must span are much larger than the colloidal
particles which make up the membrane itself. In addition, the
surface topography and electrochemical character of the substrate
can adversely affect the deposition of the particles in the
accumulating membrane on the substrate. Since the object of
depositing such a membrane on a porous substrate is to create a
material which can be used for filtering, a highly uniform size
distribution of pores in the resulting porous ceramic membrane and
a thin, uniform thickness of the membrane are desired.
SUMMARY OF THE INVENTION
The present invention is summarized in that a microporous optically
transparent membrane which has an average pore diameter of less
than 40 Angstroms is formed on a porous substrate by passing a
dilute colloidal suspension of metal oxide particles through one
side of a porous support and evaporating solvent from the
suspension by means of gas flow on the opposite side of the porous
support, so as to deposit the particles in the colloidal suspension
as a gel among the pores on the opposite side of the porous
support, followed by careful drying of the gel to form a xerogel,
and sintering of the xerogel to create a porous metal oxide ceramic
membrane.
The present invention is also summarized in that a method for
creating a particulate metal oxide ceramic membrane of defined very
small pore size includes creating a metal alkoxide in which the
alkoxyl group has a branched structure and at least four carbon
atoms, dissolving the created metal alkoxide in an alcohol solution
with a very limited amount of water, very slowly evaporating the
alcohol from the suspension thus created, and firing the resulting
gel to create a particulate metal oxide ceramic membrane having
pore sizes defined by the molarity of the metal in the beginning
alcohol and the molar ratio of water to metal alkoxide
molecules.
It is an object of the present invention to allow the creation of
metal oxide ceramic membranes in general, and titanium oxide
ceramic membranes in particular, which have a relatively small pore
size, but which can be created in an efficient and predictable
manner.
It is an object of the present invention to enable the reliable and
convenient construction of a microporous metal oxide ceramic
membrane deposited on a porous support which is useful for very
critical filtration operations, such as ultrafiltration, reverse
osmosis, and gas separation. The small-pore size membranes may also
be useful for ceramic membrane reactors, in catalysis,
photocatalysis, and in sensor and waveguide applications.
It is yet another object of the present invention to provide a
process having great utility in ultra-filtration, reverse osmosis,
gas separations, and other separation technologies in offering
significant advantages over use of other prior membranes used for
these purposes at present.
It is another object of the present invention in that it does not
involve difficult or costly equipment and can be readily adapted
for most manufacturing operations.
Other objects, advantages and features of the present invention
will become apparent from the following specification when taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic illustration illustrating the concept of the
process of the present invention.
FIG. 2 is another schematic illustration of the concept of the
process of the present invention.
FIG. 3 illustrates one embodiment of an apparatus which may be used
to perform the process of the present invention.
FIG. 4 illustrates another embodiment of an apparatus which may be
used to perform the process of the present invention.
DETAILED DESCRIPTION OF THE INVENTION
The present invention is directed to a process for gas separations,
ultra-filtration, reverse osmosis and the like using a preparation
of a ceramic metal oxide membrane from a colloidal suspension
("sol") formed of very small particles. The membranes formed from
such a sol have very small pore diameters of less than 40 Angstroms
because the rate of formation of, or the ultimate size of, the
metal oxide particles in the sol is carefully limited. The present
invention is also directed toward the creation of a microporous
metal oxide particulate ceramic membrane coated onto porous
supports which can be used as an excellent filter for separation of
small molecules either in gas or liquid phases.
The process by which the particles forming the microporous ceramic
membrane may be deposited on the porous substrate is referred to
here as permformation. The term permformation is a coined term,
combining "formation" with "permeation," to describe the process by
which the particles ultimately forming the membrane are deposited
on the support. In its most general terms, a colloidal suspension
or sol of metal or metal oxide particles is passed through a porous
support. On the far side of the porous support a gas stream causes
the evaporation of the solvent and the deposition of the particles
in the pores of the support. Capillary action continues to draw the
colloidal suspension into the support as the solvent evaporates.
The result is the deposition of a layer of colloidal particles in
the pores of and/or on the surface of the porous substrate,
adjacent to the interface where the gas stream is causing the
solvent to evaporate. This process of preferential deposition
allows one to directly control the thickness of the resulting gel
film, by varying the concentration of the sol and the rate of
evaporation of the solvent, and by controlling the temperature and
relative humidity of the drying gas. If the structure of the porous
support is isotropic, the thickness of the gel will be uniform
within the entire drying surface, and the thickness can further be
controlled by the length of time that the process is performed.
Subsequent controlled drying of the deposited gel film particles in
the face of the porous support, and firing of the gel, can lead to
a crack-free ceramic membrane of uniform thickness and of uniform
porosity in a reproducible, reliable and efficient manner.
FIG. 1 illustrates the general concept of permformation. The dilute
sol is placed on a first side of the porous support indicated at 10
in FIG. 1. The porous support 10 is, in the first embodiment
described herein, a hollow cylinder. Since a section of the porous
support 10 is viewed in cross-section in FIG. 1, two opposite
sections of the support are visible, with the sol located on the
outside of the support and the gas stream passing vertically in the
hollow center. The dilute sol is drawn by capillary flow, indicated
by the arrow 12, through the porous support. On the opposite side
of the porous support, in its center, a flow of a gas stream is
directed as indicated at 14. The gas stream can be a drying gas
such as a stream of air or, to prevent unavoidable reactions, an
inert gas such as nitrogen, or one of the noble gases. The gas
stream can also be a stream of a reactive gas, such as H.sub.2 S or
NH.sub.3, which would also cause deposition of the metal particles
as well as evaporation of the solvent. The gas stream and the vapor
from the solvent is exhausted and the particles remaining from the
dilute sol are deposited in and on the far face of the porous
support. As indicated in FIG. 1, which is a cross-sectional view of
a porous tube used as a support, the membrane may be deposited on
both interior surfaces of the cylindrical porous support.
Illustrated in FIG. 2 is a detailed schematic diagram intended to
convey the conceptual context of the permformation ceramic membrane
product made by the present invention as used with a particular
support. The method is intended to deposit a microporous membrane
on one surface of a support which is already porous. The
cylindrical porous support which has been utilized for the examples
described below is itself composed of several layers of particulate
materials, which have been sintered into a unitary material. In
this example, the support material is made of alumina particles
which have been slip-cast in a series of layers of particles of
varying size. The particular porous support used is formed of three
layers of alpha-alumina of varying size ranges. This porous support
is indicated at 20 in FIG. 2. This porous support is available as a
cylindrical assembly from United Filter. The alpha-alumina support
is composed of three layers of varying particle diameter and pore
size. The largest layer has a thickness of 1.6 mm and a pore
diameter of between 10 and 15 microns. That layer, referred to as
the substrate, is indicated at 22. The second layer, denominated as
first intermediate layer 24, is approximately 0.02 mm thick and has
a pore diameter of 0.8 microns with a porosity of 40%. The third
layer, denominated the second intermediate layer 26 here, is the
innermost layer on the tubular support, and consists of a 0.006 mm
layer of particles deposited so as to have a 0.2 micron pore
diameter therebetween. The porosity of this layer is approximately
35%. It is the object of the present invention to deposit an even
finer layer of microporous ceramic material on such a support. The
microporous ceramic membrane layer is indicated at 28 in FIG. 2.
The microporous ceramic membrane layer 28 is formed within, and
perhaps extending to the surface of, the second intermediate layer
26. The microporous membrane 28 can be thought of as a series of
very small particles deposited as a matrix or web in the pores of
the inner portion of the second intermediate layer 26. The layer 28
thus includes both the particles of the support with 0.8 micron
pores therebetween, and the microporous ceramic membrane deposited
in the pores to reduce the mean pore diameter to the range of 5 to
40 Angstroms. It is this ultrafiltration, gas separation, or
reverse osmosis layer 28, which is deposited by the method utilized
in the present invention.
While the three-layer alpha-alumina support described is a
particularly advantageous one for use within the practice of the
present invention, other porous supports may also be used. Other
porous supports which are readily amenable for use in the
permformation method include stainless supports, sintered metal
supports, porous glass (such as Vycor), fibrous mats, or one of a
line of ceramic filters sold under the Anotec trade name. The
porous support thus does not have to itself be formed of sintered
particles. While one embodiment here utilizes a cylindrical
support, many other physical configurations of the porous support
are possible, such as the flat plate in the alternative embodiment.
The apparatus for performing the process must be modified,
depending on the shape of the support, so that the sol is on one
side of the support and the gas stream is on the other side.
Shown in FIG. 3 is a first embodiment apparatus useful for
performing the permformation in accordance with the present
invention. In FIG. 3, the reaction vessel is indicated at 30. A gas
stream enters through an input port 32 and the gas stream exits,
together with the vapor of the solvent, at an exit port 34. A
stopper 36 seals the interior of the reaction vessel 30 to the
atmosphere. Within the reaction vessel 30, connected to suitable
tubing to the input and the output ports, is the cylindrical porous
support, indicated at 38. A U-shaped glass fitting 40 is located at
the bottom of the apparatus, and pieces of Tygon tubing are
indicated at 42, to connect to the input output ports 32 and 34.
Tygon is a convenient material, but any tubing impermeable to the
solvent will suffice. The appropriate quantity of sol is placed in
the reaction vessel 30, filling up the vessel to the neck thereof.
To operate the vessel, the gas stream is continually supplied
through the input port 32, and exhausted through the output 34,
thereby slowly drawing down the sol by evaporating the solvent
therefrom. As the solvent evaporates, the metal oxide particles are
deposited on the inner face of the porous support 38. In this
embodiment, the exterior of the cylindrical support 38 serves as
the sol side of the support and the interior face of the support
acts as the deposition side.
Other embodiments of the apparatus for performing the process are
also possible. For example, a variant of the reactor of FIG. 3 has
been assembled in which the cylindrical porous support 38 is
oriented horizontally rather than vertically, so that fluid
pressure drop differences over the support are minimized. Also,
shown in FIG. 4 is an alternative apparatus for performing the same
process with a flat disk-shaped porous support. In the apparatus of
FIG. 4, the reaction vessel is indicated at 130. The input gas
stream enters through input port 132 and exits through output port
134. The porous support, in this case a porous clay ceramic disk,
is indicated at 138. The input gas stream passes through flared
tubing 140 until it exits through a glass frit 142 so as to be
diffused over the top surface of the support 138. The sol is placed
in the reaction chamber 130 which is filled until the bottom of the
support is touched by the sol. A graduated leveling chamber 144
permits the sol level to be measured and provides an inlet to add
more sol if desired.
In operation, the apparatus of FIG. 4 functions analogously to the
apparatus of FIG. 3. The input air stream contacts the upper or
deposition side of the support. The sol contacts the lower, or sol,
side of the support and is drawn into the support by capillary
action. The air stream evaporates solvent on the deposition side of
the support thereby depositing the colloidal particles as a gel in
the interstices of the support.
It is also envisioned that manipulation of the porous support
and/or the gas stream may be appropriate in some instances to
achieve good membrane formation at the desired location. Since the
sol enters the support from the sol side, care must be taken to
prevent deposition of the particles until they reach the deposition
side. Therefore, charge attractions between the support and the
particles must be minimized. Additional dilution of the sol may
also help with this problem. Once support-to-particle attraction is
minimized through the support, care must be taken to ensure that
deposition occurs as desired on the deposition side. Phosphate
treatment of the deposition side may aid in forming charge
attraction at that face. The gas stream can include a reactive gas,
such as H.sub.2 S or NH.sub.3, which would change the pH of the sol
at the deposition face and thus accelerate deposition of particles.
The gas stream could be heated to destabilize the particles in the
colloid kinetically to induce deposition. Any or all of these
techniques may aid in obtaining better particle deposition
preferentially on the deposition side of the support.
In preparing metal oxide membranes using a process that involves
first creating a sol, dewatering the sol to a gel, and then
sintering the gel into a membrane, the creation of very small
particles in the sol by limiting the rate of formation or the
ultimate size of the metal oxide particle in the sol is an
important factor.
The size of the colloidal particle is the major factor in
determining the pore size of the permformed membrane. The thickness
of the permformed membrane is determined by the particle size and
by the length of time of operation of the process. The Huckel model
of an electric double layer "thickness" can be used to estimate the
effective particle size. From that size and from the knowledge of
the number of metal oxide ions in the sol, the thickness of the
resulting xerogel and membrane can be approximated in theory.
The diameter of the particles in the sol can determine the diameter
of the pores between particles in the membrane, since the model for
the microscopic structure of the membrane is a series of particles
of generally spherical shape, which are fused to their neighbors
during the sintering process, to form the porous ceramic membrane
material. Accordingly, the diameter of the pores is determined by
the diameter of the particles, since in a random close packing
model of the particles in the membrane, the smaller the particles,
the smaller are the pores formed by the gaps or spaces between the
particles. The use of the large alcohol group in the metal alkoxide
precursor, as described here, is but one method which seems to
facilitate the creation of very small particles in the sol stage by
limiting the reaction rate of the creation of metal oxide molecular
intermediates and thus limits the creation of large particles in
the sol. In particular, alcohols of at least four (4) carbon atoms,
with a branched structure, such as tert-amyl alcohol, are
preferred. The production of small pore ceramic metal oxide
membranes is also possible by processes other than those involving
large organic alcohols described herein. For instance, such
membranes may be formed by dialyzing colloidal suspensions or from
very dilute sols which discourage particle-to-particle
aggregation.
There is considerable flexibility available with respect to the
chemical composition of the sol which is used within the
permformation process described herein. Both aqueous and alcoholic
sols may be used in the permformation process described here. In
addition, the range of available metals and metal oxides is wide as
well. Metal oxide ceramic membranes can be made with titania,
zirconia, and other transition metal oxides, as well as silica,
alumina, and iron oxides. Colloidal metal particles such as
tungsten and silver may also be used.
In general, a process for producing alcoholic colloidal sols and
membranes with very small pore sizes begins with the creation of a
metal alkoxide in which the alcohol moiety in the metal alkoxide is
a large, relatively complex, organic alcohol. It has been typical
in prior art methods to utilize the commercially available forms of
metal alkoxides. For example, one convenient titanium alkoxide
commercially available (Aldrich) is titanium tetra-isopropoxide
(Ti(OPr.sup.i).sub.4). It has been found here that the substitution
of the alcohol in the beginning titanium alkoxide, by substituting
tert-amyloxide for isopropoxide, facilitates the creation of small
particles in the sol and therefore smaller pore sizes in the
resultant titanium ceramic membrane. A similar result has been
demonstrated for zirconium as well. Since the phenomenon appears
attributable to the relatively large physical size of the alcohol
moiety, it would appear that the phenomenon attributable to the
creation of the small particle sizes is the effect of the large
alcohol moiety in controlling the reaction rate of the creation of
metal oxides in the solution by interfering with access to the
titanium atom. Accordingly, other large organic alcohols,
particularly those of branch shape and having at least four or five
carbons, would result in similar control of the reaction rate, and
result in the ability to achieve small particle size and small pore
size in the membrane.
Since neither titanium nor zirconium tetra-tert-amyloxide is known
to be commercially available at present, they must be created from
readily available materials. One convenient method for creating
titanium or zirconium tetra-tert-amyloxide is by an alcohol
exchange method, using the commercially available precursor
tetra-isopropoxide. This is done by reacting the titanium (or
zirconium) tetra-isopropoxide with tert-amyl alcohol in a benzene
solvent to yield titanium tetra-tert-amyloxide and isopropanol.
Then by distillation, isopropanol can be removed with the benzene
as an azeotrope, and then excess tert-amyl-alcohol and benzene can
be removed by distillation.
Once the metal tetra-tert-amyloxide is available, the reaction may
proceed. The metal alkoxide and a small amount of water are
separately dissolved in equal amounts of alcohol, with the alcohol
preferably being the same organic alcohol that is the alkoxyl group
in the metal alkoxide. The three other critical parameters appear
to be the molar concentration of titanium in the ultimate solution,
the molar ratio of water molecules to metal atoms, and the pH of
the water. These three parameters are interdependent. Thus for a
molarity of metal molecules of 0.2 molar, the ratio of water
molecules (pH=2) to metal atoms has been found to be conveniently
in the range of 1 to 7 to achieve desirable membranes. However, for
higher metal concentrations, i.e. a molarity of titanium of 0.4
molar, then a ratio of water molecules (pH=2) to metal atoms should
not exceed about 3 before the reaction ceases to function
effectively. The pH of the water is another factor which affects
the particle size formed in the sol since the protons act as a
catalyst for the hydrolytic reactions. Values of pH in the range of
about one to three are preferred, with a lower pH generally
resulting in smaller particle size. If the water ratio is too high,
or if the molarity of the metal atoms becomes too high, the
creation of metal oxide molecules becomes prevalent in the solution
and a precipation results, which yields particles of a size higher
than is desired in creating the membrane here. However, by limiting
the molarity of the metal and the ratio of water to metal, and by
adjusting the pH of the water, the size of the particles can be
strictly limited in a way that results in efficient creation of
finely porous membranes.
Again, the procedure begins with the dissolving of the titanium or
other metal alkoxide and water in separate amounts of the alcohol.
The two solutions are then mixed together by dropping the water
alcohol part into the alkoxide part. The transparent solution
resulting is preferably stirred while the reaction continues. This
step may require some time since the reaction rate of the formation
of metal oxides has been impeded deliberately in order to prevent
the creation of large particles. The result is a transparent
solution containing very small suspended metal dioxide
particles.
As has been published previously by the inventors of the present
specification, it is possible to create both polymeric and
particulate titanium ceramic membranes which are porous, stable,
and can be made generally crack-free. What has been surprisingly
found here is that a process can be defined for making particulate
ceramic membranes of very small defined pore diameter using a
process that bears much more similarity to the previous process for
creating polymeric titanium ceramic membranes than that previously
used to create particulate ceramic membranes of titanium. This
procedure for making small pore membranes omits the peptizing step
normally associated with the creation of particulate metal oxide
ceramic membranes. The small particles here are formed directly
from the hydrolytic reaction by using a limited amount of water.
However, this alcoholic sol still results in a particulate
membrane, presumably since the large alcoholic group in the metal
alkoxide precursor prevents the partially hydrolyzed intermediates
from polycondensation which is the key route to forming polymeric
chains.
While the method for making small pore size membranes and the
product disclosed herein are illustrated in particular with metal
oxide ceramic membranes of titanium, zirconium, and a mixture of
the two, it has been described previously by others in the field
that methods proven to be effective with titanium may also be
adapted for use with other metallic oxides, such as oxides of
silicon, aluminum, niobium and other transition metals. Thus the
method and product of the present invention has utility for other
metals as well, although titanium is considered one of the more
difficult metals to work with, of the metals useful for creating
metal oxide ceramic membranes, and titanium has particularly unique
qualities advantageous in a metal oxide membrane, due to its
catalytic and photocatalytic characteristics, not present in some
other metal oxides which might also be used in such a membrane.
A procedure for forming aqueous colloidal sols involves producing
metal hydroxide particles from metal alkoxides by hydrolysis in an
acidic aqueous solvent with a basic chemical such as NaOH or
NH.sub.4 OH. After stirring the hydrolyzing metal into a homogenous
sol, the non-metal hydroxide products are removed by washing or
dialysis. Metal hydroxide particles remain in suspension in very
acidic aqueous solutions.
To convert the resultant alcoholic or aqueous sol into a gel, the
solvent must be removed from the solution. However, the process
must be delicately handled in order to avoid concentrating the very
small particles into larger particles than is desired. It has been
found that slow evaporation in a humidity controlled box is a
sufficiently slow process to produce clear gels at room
temperature. The dried gels then can be fired in air at
temperatures not to exceed 400.degree. C. to sinter the gel into a
titanium dioxide particulate membrane. It has been further found
that during firing the heat of the sintering oven must be raised
very slowly in order to prevent carbon deposition on the membrane
during firing.
The precursor sols can be tested for the particle size by
quasi-elastic light scattering techniques. Using such a technique
it has been found that the particles in the sol have a diameter
which can be varied down to less than 5 nanometers. The use of
transmission electron microscopic imaging of the dried gel has
revealed that the dried gel is composed of quantum sized particles
having a size of less than 3 nanometers. BET measurements of the
resulting membrane fired to 250.degree. C. have indicated that
membranes can have a mean pore diameter as small as 14 Angstroms
with an extremely narrow distribution of pore size. The BET results
also indicate that a large surface relatively low porosity (39%)
relative to the theoretical area (in excess of 200 square meters
per gram) and a low porosity of about 30% can be achieved in such a
membrane. By altering the ratio of water to metal and by altering
the molarity of metal in the beginning solution, the diameter of
the particles in the solution, and the resulting diameter of the
pores in the membrane, can be controlled between 5 and 40 Angstroms
in a relatively efficient manner. It has also been found that
polymeric gelation is completely prevented by the steric effect of
the large alcohol group on polycondensation reactions. Particles of
different size ranges, in the range of 2 nanometers (20 Angstroms)
to 300 nanometers, can be harvested by quenching the particle
growth at certain stages using polymeric stabilizing agents, such
as polyethylene glycol and hydroxypropyl cellulose. In this fashion
tailor-made membranes with desirable pore sizes throughout the
range can be obtained by gelatinizing corresponding particle sols.
The lower range limit on the size of the pores in such a membrane
is difficult to ascertain due to difficulties in measurement of the
pores, but particles sized so as to give rise to pores as small as
5 Angstroms in diameter have been achieved.
In order to maintain preferential deposition of the colloidal
particles at the mouth of the support pore, the interactions
between the particles and the support walls, and interactions
between the particles themselves, must be kept to a minimum until
the particles reach the surface where deposition is desired. It is
for this reason that a dilute sol may preferably be used in order
to minimize interactions between the particles themselves. A sol
which is sufficiently dilute such that the average spacing between
particles is significantly larger than the size of the particles
themselves achieves this objective. Using an orthorhombic (8
nearest neighbor) configuration as a model for the distribution of
sol particles within the sol, it is possible to determine the
average separation distance between the nearest neighbor particles.
For example, the molarity necessary to achieve a required
separation distance between the particles has been calculated in
the following Table 1. The particle spacing factor is designated
"n" and the molarity necessary to achieve an n of 1, 5 or 10 is
disclosed for two diameters of particles. The results are given in
terms of the molarity necessary to achieve the desired separation
of particles to avoid these interactions.
TABLE 1 ______________________________________ Molarity of Sol to
Achieve Particle Separation Diameter of pH particles (nm) n = 1 n =
5 n = 10 ______________________________________ 8 25 1.4 0.011
0.0014 2 12 12 0.098 0.012
______________________________________
Following the permformation procedure, the deposited gelled
colloidal particles must be dried to form a xerogel. This is done
by critically slow drying, to remove the remaining solvent
contained within the xerogel without cracking it. To reduce the
drying rate in the bore of the support, i.e. at the gel surface,
the configuration of the permformation may be reversed. The glass
U-tube 42 at the bottom of the apparatus is filled with solvent,
and the inlet and outlet ports 32 and 34 are then sealed. This
procedure is intended to result in a 100% relative humidity
environment inside the drying loop. The sol reservoir is then
emptied, and left open to ambient humidity conditions. The relative
humidity gradient thus imposed across the wall of the support is
the reverse from that experienced during the permformation process.
This relative humidity gradient imposed across the support causes
the meniscus of the sol to recede toward the outer surface of the
support. A typical drying time would be one to two days.
The dried xerogels can then be fired in ambient air conditions.
Firing conditions for the supported membranes typically involve a
relatively gradual heating rate of 2.degree. C. per minute until a
maximum temperature of 400.degree. C. is reached. Previously
experiments have indicated that using mixed metal oxide ceramic
membranes, firing temperatures of up to 600.degree. C. can be used
for some membranes, though typically firing ranges between
400.degree. and 600.degree. C. are common. The tube is maintained
at the peak firing temperature for a time period, typically four
hours, and then is cooled to room temperature again in a controlled
rate of descent of approximately 2.degree. C. per minute.
The result of the performation process is a microporous metal oxide
ceramic membrane deposited on the support which gives the material
great strength and rigidity. The microporous membrane is actually
deposited within the pores of the support and perhaps extending
over the deposition side of the support as well. The material thus
formed is suitable for fine filtration operations, notably for
ultrafiltration, reverse osmosis, gas separation, and molecular
sieving. Since the size of the pores can be readily manipulated
within a narrow range, by tightly controlling the size of the
particles used to form the membrane, permformed membranes can be
designed and constructed according to desired specification. Such
materials can be used for gas separations, liquid filtrations, and
separations of materials from solvents, such as desalination of sea
water. The materials can also be used in catalytic membrane
reactors and for catalysis in general.
Titanium oxide and other metal oxide porous ceramic membranes
containing small pores, with a relatively narrow distribution of
pore size, offer several unique advantages for industrial
application. Because the metal oxide ceramic materials are highly
durable, the membrane is an attractive candidate for carrying out
high pressure reverse-osmosis type processes, such as producing
ultrapure water and the desalinization of sea water. Since it has
been previously demonstrated that titanium in a titanium oxide
ceramic membrane retains its catalytic ability, such a titanium
oxide ceramic membrane can be used both as a catalyst or catalyst
support and can speed up certain reactions by removing unwanted
by-products due to the separation functions. It has previously been
demonstrated that membranes of this type are photochemically active
and are capable of degrading complex organic molecules such as
polychlorinated biphenyls and other environmental contaminants and
of separating gaseous mixtures.
The process of the present invention can also be better understood
with reference to the following examples which are intended to be
illustrative and not limiting.
EXAMPLES
Particulate TiO.sub.2 Membranes
First, a supply of the precursor tert-amyloxide was prepared from
commercially available materials. As is shown in Equation 1,
titanium tetra-isopropoxide (Aldrich) was converted by an alcohol
exchange reaction to titanium tetra-tert-amyloxide. This reaction
was conducted by the method described by Bradley, et al., J. Chem.
Soc., 2027 (1952). The titanium tetra-isopropoxide was reacted with
tert-amyl alcohol (t-AmOH) (Aldrich) in a benzene solvent to yield
titanium tetra-tert-amyloxide and isopropanol (i-PrOH). The
isopropanol was then removed from the solution by distillation with
benzene as an azeotrope at 71.4.degree. C. The removal of the
isopropanol was believed necessary to complete the formation of the
tetra-tert-amyloxide. Excess t-AmOH and benzene were then removed
via additional distillation at above 100.degree. C. The NMR
spectrum of the resulting light yellow product was taken to confirm
that no isopropanol remained. There may have been trace amounts of
t-AmOH in the product. ##STR1##
All other chemicals were used without additional purification.
The preparation of the sol was begun by dissolving the titanium
alkoxide and acidified water in equal amounts of alcohol. The water
was previously acidified to a pH of 2. The desired concentration of
the titanium in the solution and the molar ratio of water to
titanium in the solution were calculated in advance. Based on a
calculation of a molar level of 0.2 M of titanium and a molar ratio
of water to titanium of 6 to 1, 1.19 grams of Ti(OAm.sup.t).sub.4
were dissolved in 7.5 ml of AmOH while 324 .mu.l of H.sub.2 O was
introduced into 7.5 ml of AmOH as well. The water fraction was then
introduced into the titanium alkoxide fraction by dripping while
stirring. The dripping occurred over a fifteen minute time interval
at room temperature. While the resulting solution appeared visibly
transparent, light scattering measurements indicated small
particles (having a diameter less than 5 nm) in suspension. The
solution, which was 0.2 M titanium tetra-tert-amyloxide and 1.2 M
H.sub.2 O, was stirred during an aging time of an additional two
hours, also at room temperature.
To turn the sol thus produced into a gel, the alcohol was slowly
permitted to evaporate from the sol. The solution was placed in
plastic dishes for the gelation which was accomplished by placing
the dishes in a humidity controlled box, which was simply a
conventional desiccator box without either particular
instrumentation or mechanism to control the alcohol level in the
chamber. After one week, the dishes were checked. Some produced
satisfactory gels after one week while others required a longer
gelation time. To avoid cracking, the gels were allowed to dry
completely before being removed from the box.
The dried gels were then sintered by firing in a ceramic oven in
air at up to 400.degree. C. The temperature rise in the oven was
controlled to be no more than an increase of 0.1.degree. C. per
minute, at least in the temperature range of 190.degree. C. to
350.degree. C., in order to prevent carbon deposition during the
firing process. The final firing temperature, i.e. 400.degree. C.
was held for about one-half hour. The results were unsupported
titanium ceramic membranes which were visibly transparent.
Various measurements were made to gauge the size of the particles
during the formation process to gauge the size of the pores in the
membrane. The precursor sols were tested by a quasi-elastic light
scattering technique and were found to contain TiO.sub.2 particles
which were less than 5 nanometers in diameter. Transmission
electron microscopy of the dried gel revealed that the gel was
composed of quantum-sized particles, of less than 3 nm in diameter.
BET measurement indicates that one of the membranes, fired at
250.degree. C., has a mean pore diameter of 14 Angstroms with an
extremely narrow distribution of pore size. The BET measurement
also showed a large specific surface area, i.e. 264 square meters
per gram, and relatively low porosity, i.e. 39%.
The same procedure was repeated with the same molarity of titanium
in the solution and molar ratios of water to titanium of from 1:1
to 10:1, with best results being obtained at 6:1 for 0.2 M
titanium. Ratios in the range of 2:1 to 7:1 yielded reasonable
results. With higher levels of water present, the titanium dioxide
tended to precipitate, resulting in larger than desired particle
size. For 0.4 M titanium, and varied ratios of water to titanium,
it was found that water to titanium ratios in excess of 3:1
resulted in precipitation. This critical and interrelated factor
appeared to be both the molarity of the titanium and the ratio of
water to titanium, with the water ratio needing to be lower if more
titanium was present. This observation is consistent with the model
that the system is effective in creating small particle size, and
small pores, due to the limitation in the availability of the water
molecules to the titanium atoms and in the titanium particles to
other titanium particles.
Particulate ZrO.sub.2 Membranes
Again a supply of the precursor tert-amyl oxide was prepared by an
alcohol exchange reaction from commercially available zirconium
tetra-propoxide (Zr(OPr.sup.n).sub.4) (70% propanol, Aldrich) with
tert-amyl alcohol (Aldrich) in benzene solvent to yield zirconium
tetra-tert-amyloxide and propanol. By distillation, propanol was
removed with benzene as an azeotrope at 77.1.degree. C. Complete
removal of propanol was desired. Excess benzene was removed by
subsequent distillation over 80.degree. C. The yellow solid product
was tested by NMR spectrum and found to have in excess of 95% of
the propoxyl groups replaced by tert-amyl groups. Thus the equation
paralleled equation 1, above, with zirconium substituted for
titanium. The product was then mixed with t-AmOH to make a 1.1 M
solution.
The zirconium alkoxide and water were separately dissolved in equal
amounts of tert-amyl alcohol. The two solutions were then mixed by
dropping the H.sub.2 O part into the alkoxide part over 15 minutes.
Two concentrations of solution were made, one 0.2 M
Zr(OAm.sup.t).sub.4 and 0.2 M H.sub.2 O and the other 0.1 M
Zr(OAm.sup.t).sub.4 and 0.2 M H.sub.2 O. The solutions were aged by
stirring for two hours.
The transparent sols thus produced were then placed in plastic
dishes for gelation, which was accomplished by slow alcohol
evaporation carried out for about one week in a desiccator box. The
gels were then fired in air at up to 400.degree. C. to result in a
transparent unsupported ZrO.sub.2 membrane. The hydrolyzed clear
sols were also used for coating a glass support. A transparent
crack-free film seven layers thick, which had a total thickness of
about 1 micron, was obtained using a spin-coating technique.
Again measurements were made to ascertain the size of the particles
and the pores in the membrane. The precursor sols were tested by a
quasi-elastic light scattering technique and found to contain
ZrO.sub.2 particles having sizes less than 5 nm in diameter.
Nitrogen adsorption measurements of the membranes fired between
200.degree. and 350.degree. C. indicated a mean pore diameter of 14
Angstroms. An x-ray diffraction study revealed the membrane to be
completely amorphous.
Particulate Mixed Titanium and Zirconium Membranes
Both the titanium tert-amyl alcohol and the zirconium tert-amyl
alcohol were prepared from commercial materials by the alcohol
exchange methods described above. The metal alkoxides were mixed in
ratios of 10% and 20% zirconium. The concentrations of total metal
atoms used were half the concentration of water molecules.
An unsupported Zr.sub.0.1 Ti.sub.0.9 O.sub.2 membrane was made by
stirring 540 .mu.l of 1.1 M Zr(OAm.sup.t).sub.4 solution and 1.76
gm Ti (OAm.sup.t).sub.4 into 12.8 ml tert-amyl alcohol, followed by
stirring for one hour. Separately 220 .mu.l of H.sub.2 O was
dissolved in another 15 ml of AmOH. Then the water solution was
dripped into the metal solution. The final transparent sol had 0.02
Zr, 0.18 Ti and 0.4 M H.sub.2 O. The solution was stirred for two
hours. The sol was then poured into plastic dishes and placed in a
desiccator box for three weeks. The gels were then removed and
fired by slow temperature increase (2.degree. C./min) up to
400.degree. C. and baked at 400.degree. C. for one-half hour. The
resulting unsupported membranes were transparent. Testing of the
sols by quasi-electric light scattering technique indicated Zr-Ti
oxide particles having sizes less than 5 nm. The membrane was
measured by nitrogen adsorption and found to have a mean pore
diameter of less than 16 Angstroms with an extremely narrow pore
size distribution. BET results revealed a large surface area of
200-300 m.sup.2 /gm and a porosity in the range of 30-35%,
consistent with the close packing model. X-ray diffraction revealed
the membranes to be amorphous.
Supported Zr.sub.0.1 Ti.sub.0.9 O.sub.2 membranes were made on a
glass substrate by first dissolving 1 ml 1.1 M Zr(OAm.sup.t).sup.4
and 3.31 gm Ti(OAm.sup.t).sub.4 into 10.6 ml tert-AmOH, followed by
stirring for one hour. Separately 414 .mu.l of H.sub.2 O were
dissolved in another 15 ml AmOH. The water solution was dripped
into the metal alcohol solution with violent stirring. The final
transparent sol contained 0.038 M Zr, 0.34 M Ti, and 0.76 M H.sub.2
O. The solution was stirred for two hours and then coated onto
prewashed microscope slides of size 1.3.times.4 inch (VWR
Scientific). One side of the glass slide was coated by spin
coating, followed by firing at 200.degree. C. for 15 minutes. Up to
seven layers of coating of 1 micron each were made without
cracking. Finally the membranes were fired at 550.degree. C. for
one hour to make a transparent porous membrane.
The Formation of an Iron Sol
The fabrication of a microporous supported ceramic membrane was
begun with the consensus of a metal oxide colloidal solution or
sol. The fabrication of an iron oxide ceramic membrane was begun
with goethite, which was synthesized from ACS reagent grade
chemicals and Milli-Q deionized water. To synthesize the goethite,
a solution of ferric nitrate (125 ml, 0.83 M) was passed through a
glass microfiber filter to remove dust and undissolved
particulates. The ferric nitrate was then partially neutralized by
adding NaOH (41.6 ml, 5 M) with rapid stirring. The OH to Fe ratio
was calculated to be 2.0. Following some initial precipitation, the
ferric nitrate solution resolubilized after about 30 minutes. The
ferric nitrate solution was then aged in a shaker at 25.degree. C.
in a glass container for 60 hours. After aging, the pH of the
solution was 1.4. The partially neutralized ferric nitrate solution
was then hydrolyzed by the addition of NaOH (30.2 ml, 5 M), which
was added over a 3 minute period in a polypropylene container with
vigorous stirring by a Teflon impeller. The pH of the iron solution
was thus increased to 12.6 in about 3 minutes. The hydrolyzed iron
solution was then aged in a shaker at 60.degree. C. for 6 days.
Initially, the color of the iron solution was a dark reddish brown,
but after 24 hours in the aging period, the color changed to a
light, orange-tan which is indicative of the formation of goethite
(FeOOH) particles. Excess electrolytes were removed from goethite
sol by repeated washings with Milli-Q water followed by settling
and decanting. The washing was continued until no further decrease
in the conductivity of the supernatant could be detected. The
goethite sol was then ready for use in the permformation
procedure.
The Formation of Silica Sol
An aqueous silica sol was synthesized from ACS reagent grade
chemicals in Milli-Q deionized water. The process was begun with
4.5 ml of tetraethyl orthosilicate (TEOS) which was added drop-wise
to NH.sub.4 OH solution (31 ml, 0.5 M) with rapid stirring.
Initially, a two-phase mixture was formed, but after stirring for 1
hour the solution became a homogenous silica sol. The sol was
transferred to a dialysis membrane (3500 molecular weight cut off)
to remove ammonium ion and ethanol which had been formed during
hydrolysis. The sol was dialyzed against Milli-Q water until the pH
of the sol dropped to below 9. The purified sol was then filtered
using glass microfiber paper to remove any dust or particulates.
The aqueous silica sol was then ready for use in the permformation
procedure.
Formation of Performed Membranes from Aqueous Sols
Both the silica and iron membranes were formed in the apparatus of
FIG. 3. The sol was placed inside of the reaction vessel 30. The
assembly including the input port and output ports 32 and 34, the
porous support 38, the U-shaped fitting 40 and the Tygon tubing 42
was placed as a unit into the reaction vessel, with the stopper 36
sealing the vessel to the atmosphere. A seal was made between the
Tygon and the porous ceramic using epoxy resin. Where the ends of
the ceramic support were glass-glazed, the epoxy sealant was not
used.
A length of nylon thread was inserted between the stopper and the
cylinder in the neck to allow pressure equalization as the sol
level dropped. In order to minimize subsequent loss of vapor
through the neck of the reaction vessel, a paraffin film was
wrapped around the stopper joint.
High purity nitrogen gas was used as the drying medium. The
nitrogen cylinder and the regulator were attached to the inlet port
32 by a length of Tygon tubing. On the output port 34, 2 humidity
indicator cards served as a rough estimate of the humidity of the
gas flow stream relevant to ambient conditions.
The length of the permformation operation was determined by
measuring the decrease in sol level over time. An average sol
evaporation rate was calculated as the change in volume over time
for each time period. Based on models of the support pore
structure, and the packing of the colloidal particles during
gelation, an approximate membrane thickness was calculated. Tables
2 and 3 below set forth the results achieved with the silica sol
when deposited through the permformation procedure. The first run
was conducted with a highly dilute silica sol (separation factor of
20). The run lasted 27 hours and was intended to produce a 3 micron
thick membrane. The second run was conducted with a more
concentrated sol (separation factor of 10) and was intended to
produce a membrane with a thickness of 8 microns.
TABLE 2 ______________________________________ Dilute Silica Sol
Level of Rate of deposition Thickness Time sol (mm) (ml/hr) (.mu.m)
______________________________________ 0.0 42 16.5 25 0.84 2.1
20.75 20.5 0.86 2.6 26.5 15.0 0.78 3.3
______________________________________
TABLE 3 ______________________________________ Concentrated Silica
Sol Level of Rate of deposition Thickness Time sol (mm) (ml/hr)
(.mu.m) ______________________________________ 0 39 -- 0 2 37 0.82
0.5 22 25 0.49 3.6 24 23.5 0.61 4.0 32.5 19 0.43 5.2 45 12.5 0.42
6.8 51.5 9 0.44 7.7 ______________________________________
Following the permformation procedure, the gelled colloidal
particles were dried to a xerogel and fired to create the sintered
porous ceramic membrane. The drying step must be done carefully to
avoid gel cracking which can be caused by evaporative stress. To
reduce the drying rate in the bore of the support, the
configuration of the permformer was reversed. The glass-tube was
filled with water and the inlet and outlet ports 32 and 34 were
sealed. This resulted in a 100% humidity environment inside of the
drying loop. The sol reservoir was then emptied and left open to
ambient humidity conditions. The relative humidity gradient imposed
across the wall of support caused the meniscus of the sol to recede
toward the bore surface of the support. Typical drying times were 1
to 2 days. After the membranes were dried, the end seals were
removed using a diamond saw.
The resulting dried xerogels were fired in the ambient air. The
firing conditions were controlled so that the heating and cooling
ramps were 2.degree. C. per minute and with a maximum firing
temperature of 400.degree. C. which was held for a duration of 4
hours.
One indication of the successful deposition of the small colloidal
particles in the porous support is that the rate of flow of sol
through the support decreases over time. It has been found that the
rate of flow of sol, as indicated by the rate of solvent
evaporation, does decrease over the time of the permformation. The
following Table 4 sets forth the decreasing rate of flow measured
for a silica sol being deposited in the cylindrical gamma-alumina
support.
TABLE 4 ______________________________________ Cumulative
Incremental Overall Time of Drop in Sol Evaporation Evaporation Run
(min) Level (ml) Rate (ml/hr) Rate (ml/hr)
______________________________________ 0 .15 -- -- 14 .43 1.20 1.20
29 .76 1.32 1.26 60 1.23 .91 1.08 109 1.80 .70 .91 133 2.00 .50 .83
225 2.60 .39 .65 386 3.52 .34 .52 438 3.80 .32 .50
______________________________________
It is expected that the microporous ceramic membranes will have
mean pore sizes adjustable in the range of from 5 to 100 Angstroms.
Because the membranes are being formed in the pores of a support,
overall porosities will be low, typically less than 30%.
Microporous membranes with pores less than 100 Angstroms may be
used for ultrafiltration while microporous membranes with pore
sizes in the 5-40 Angstrom range may be used for reverse osmosis,
molecular sieving, and other types of gas separations. Because of
the durability of ceramic materials, the membranes should withstand
significant pressure drops and be useful for industrial
applications.
Formation of Permformed Membranes from Alcoholic Sols
(Hypothetical)
Metal oxide membranes can be formed in the apparatus of FIG. 3 from
alcoholic sols such as those described in earlier examples directed
to production of particulate TiO.sub.2, ZrO.sub.2, and mixed
Zr.sub.0.1 Ti.sub.0.9 O.sub.2 membranes. Using the permformation,
drying, and sintering processes of the previous example, such metal
oxide membranes may be deposited on the porous support, dried to a
xerogel and fired to form sintered microporous ceramic membranes.
It is expected that the microporous ceramic membranes will have
mean pore diameters adjustable in the range of from 5 to 100
Angstroms and will be useful for ultrafiltration and gas
separation. Membranes with pore sizes in the 5 to 40 Angstrom
diameter range may also be used for reverse osmosis and molecular
sieving. Molecular sieving in the gaseous phase may be accomplished
by permitting a mixture of gases having various molecular radii to
flow through the permformed supported membrane. Gases with
molecular radii smaller than that of the pores in the membrane pass
through the membrane and may be recovered in a purified or
partially purified form after passing through.
* * * * *